Electrodes of electrochemical gas sensors often fulfill a dual function by being used both for the electron transfer in contact with a surrounding medium and for the catalytic conversion of gas components at their surface. In the case of ceramic sensors, it is especially difficult to achieve long-lasting and sufficient catalytic activity of the catalytically active layers functioning as electrodes. The original catalytic activity of the layers is often lost as early as in the sintering processes during the manufacture of the gas sensor; in addition, when working with highly corrosive gas mixtures, the same thing occurs during the operation of the gas sensor.
German Patent No. 42 40 267 describes that the electrodes of an exhaust gas sensor be impregnated with a solution of a catalytically active metal such as platinum or rhodium after sintering the gas sensor. In this manner, a highly active electrode is obtained despite the prior sintering process. However, this procedure does not change the insufficient stability of the electrodes during long-term operation in corrosive gas mixtures.
German Patent No. 40 33 388 describes a layer system for gas sensors where a catalytically active electrode of the gas sensor is coated with a protective layer including substances that absorb harmful contaminants. In this manner, the stability of the gas sensor electrodes is enhanced. This arrangement requires a multilayer structure.
German Patent Application No. 1 129 291 describes a Raney catalyst made of a silver alloy including silver and up to 60 wt. % aluminum. This Raney catalyst is used as an electrode for fuel elements. Raney catalysts are also used for hydrogenation and dehydrogenation reactions where mainly Raney nickel is of importance. In addition, Raney silver, Raney cobalt, Raney iron, Raney copper, and Raney molybdenum are referred to as catalysts.
It is an object of the present invention is to provide a catalytically active layer that has high and long-lasting catalytic activity.